1. Field of the Invention
The invention relates to image projectors, in particular video projectors, using several light beams of different colours to form the image to be projected. It relates especially to means for improving the luminous efficiency of such projectors.
In regard to colour image projection, greater and greater emphasis is being placed on image projection devices which implement techniques of spatial modulation of light. These techniques use electro-optical valves or screens for spatial modulation of light, termed "modulation screens" in the remainder of the description. Spatial modulation of light is to be understood to mean that a beam is projected globally onto the whole surface of an image to be projected, as opposed to temporal, or time, modulation systems in which a slender pencil of light (corresponding to an image point only rather than to the whole image) is modulated temporally at the same time as it scans the surface of a projection screen.
A spatial modulation screen usually consists of a screen of the LCD type (Liquid Crystal Display) comprising a matrix layout of liquid crystal cells, each cell corresponding to an elementary image area or pixel.
The polychrome image can be obtained either with the aid of three modulation screens each forming a monochrome image of one of the three primary colours red, green and blue, or from a single three-colour modulating screen, that is to say a modulating screen furnished with filters coloured according to the three primary colours.
In the case of projectors using a single three-colour modulation screen, at least three neighbouring elementary image areas constitute a three-colour pixel. Each of these three elementary areas corresponds to one of the primary colours, red, green and blue depending on the colour of the filter assigned thereto.
The solutions using a single three-colour modulation screen are well known, and they have the advantage of being particularly simple. On the other hand, they lead to a loss of brightness by a factor of at least three in each of the three chromatic components, this loss being imposed through the spatial distribution of the colour filters.
In the case of projectors using three modulating screens each forming a monochrome image, each modulating screen spatially modulates a beam of light coloured in accordance with a primary colour, and the polychrome image is obtained by superimposing the three monochrome images on a projection screen.
The images formed by the three modulating screens are projected onto the projection screen with the aid of a single objective which is common to the three monochrome images, or else with the aid of three objectives. The coloured beams of the various primary colours, are obtained from a single beam of white light produced by a light source termed the "white lamp", behind which are placed colour filters. Generally the white lamps used are of the arc lamp type, and more especially of the halogen type which exhibits a good luminous yield.
2. Description of the Prior Art
FIG. 1 shows diagrammatically a conventional colour image projector of the type with three modulating screens as defined above.
The projector includes a source of white light SL formed by an arc lamp 1 arranged at the focus of a reflector 2. This arrangement gives rise to a beam of white light FB of small divergence, propagating towards a modulating assembly with three modulation screens ER, EG, EB each intended to form an image having one of the primary colours; this assembly has reflecting mirrors MR1, MR2 and dichroic mirrors MD1 and MD4 with the aid of which the beam of white light FB is split into three coloured beams FPR, FPG, FPB whose spectral band corresponds to one of the primary colours red, green and blue. The dichroic mirrors are wavelength-selective mirrors, that is to say they transmit a certain spectral range and reflect the complementary range.
The three coloured beams each pass through a modulating screen ER, EG, EB by which they are modulated. They are later superimposed on the same axis along which they propagate towards a projection objective OP. They pass through this projection objective OP, and the monochrome image which they each carry is projected onto a projection screen EP. The superimposing of the three monochrome images produces the three-colour image.
Represented in the figure are field lenses LC which, in a conventional manner, are arranged in the vicinity of each modulation screen ER, EG, EB. It should furthermore be noted that modulation screens of the LCD type act on the direction of polarization of the light, and consequently the light which passes through them must be polarized. A polarizer and an analyser (which are not shown), arranged respectively upstream and downstream of the modulation screen, are therefore generally required for each modulation screen.
The colour image projectors of the type using one or more screens for spatial modulation of light are of interest, in particular for their compactness, by comparison with the techniques using cathode-ray tubes. However, these projectors with modulation screens have the disadvantage of exhibiting poor luminous yield.
Indeed, in nevetherless the most favourable case of a projector with three modulation screens, the overall transmission of light is commonly estimated at around 1%, which means that only 1% of the light produced by the white light source SL leaves the projection objective OP.
The inventors have noticed that, in respect of a non-negligible share, this poor luminous yield was attributable to the lamp itself serving to produce the light, bearing in mind the necessity to comply with a given colour balance between the three primary colours, red, green and blue, which colour balance is imposed by the sensitivity curve of the eye. The lamps used are generally arc lamps of the "metal halide" type, which exibit a high luminous yield in lumens per watt. However, one of the main disadvantages of these lamps is that their emission spectrum is never strictly adapted to the colorimetry requirements, they are generally lacking in lumens in one or even two of the three primary colours.
It should be noted in this regard, that the manufacture of these arc lamps poses problems, particularly, for the obtaining, with good efficiency, of an emission spectrum compatible with the production of lumens in the three primary colours.
It may be observed ordinarily, that to obtain the colour balance between the three primary colours, there is occasion to eliminate a certain quantity of lumens from the excess primary. This results in a significant reduction in the luminous yield.
With a view to increasing the luminous efficiency of three-colour image projectors using a light beam passing through a spatial modulation system, the invention proposes to produce the light, in the three spectral bands corresponding to the three primary colours, with the aid of at least one monochromatic source. Two types of preferential embodiment are envisaged: in one, three monochromatic sources corresponding to the desired primary colours are used, each in conjunction with a respective spatial modulation screen. In another advantageous embodiment, the light source used will be a white light source with which is associated a source of monochromatic light with a primary colour wavelength for which the spectrum of the white source is especially deficient.
Up till now, it has always been considered that a source of monochromatic light, in practice a source of coherent light (laser) was suited solely to carrying out projection by temporal modulation and beam scanning over the projection screen. Indeed, the usual property of coherent monochromatic sources is the concentrating of their beam onto a very small section, which makes them especially suited to screen scanning and, conversely, ill-adapted to global projection by spatial modulation.
The U.S. Pat. No. 4,145,712 describes an image projection system with two lasers, with temporal modulation and screen scanning.
The Patent EP 0 322 070 describes a combination of three light sources with spectra centred on the blue, green and red portions respectively of the visible spectrum. These sources are not monochromatic.
It is now well known how toproduce monochromatic sources of non-negligible spatial spread which may lend themselves advantageously to projection with spatial modulation, something which was not a priori conceivable with the usual monochromatic sources owing to their narrow pencil.